[Problem] To simplify equipment for converting carbon dioxide contained in natural gas into methane, to prolong the life of a methane formation catalyst, and to reduce the use amount thereof.
[Solution] Reactors are installed in the following order from the upstream side: a hydrogenation reactor 1 in which a layer 11 of a hydrogenation catalyst has been disposed; an adsorptive desulfurization reactor 2 in which an adsorbent layer 21 for adsorbing hydrogen sulfide has been disposed; and methane formation reactors 3, 4, and 5 in which methane-formation catalyst layers 31, 41, and 51 have been disposed respectively. Hydrogen gas is mixed with natural gas, and the mixed gas is heated and fed to the hydrogenation reactor 1. Thus, the organic sulfur compounds contained in the natural gas are hydrogenated by means of the hydrogenation catalyst to become hydrogen sulfide, and a methane formation reaction takes place. In the methane formation reactors 3 and 4, hydrogen is fed to the desulfurized natural gas to cause a methane formation reaction. Furthermore, in the succeeding-stage reactor 5, the hydrogen concentration in the gas is regulated. Since hydrogenation and methane formation are conducted as pretreatments, the sulfur concentration can be kept extremely low and the amount of the methane formation catalyst to be used can be reduced.